I edited my post. The box art links are working now again. They were definitly NOT working about 10h ago, for several hours. Giving https://static-cdn.jtvnw.net/ttv-static/404_boxart.jpg errors. I noticed with my app that new box are links for games shown in my apps were not working anymore, I thought you changed the valid width and height maybe, but then I noticed ALL box are links gave a 404.
Does this have something to do with the timing of Legacy badges endpoint shutdown details and timeline - June 2023 - #4 by jbulava ?
Maybe you should investigate this.